My U.S. Pat. No. 4,504,733 discloses and claims a closed loop temperature control method and system now in use in incubator control. The control system described in the patent does not include protection against failure of the element (thyristor 30) directly controlling the flow of electrical heating current. If the thyristor fails by becoming a short triac branch of the circuit, the result may be catastrophic, because only a relatively slight rise in temperature can spoil the reaction in the incubator.
Many closed loop temperature control systems guard against catastrophic temperature runaway by utilizing a second temperature control system which is set at a slightly higher temperature than the first and which "takes over" if the temperature exceeds that for which the first system is set and equals that for which the second system is set.
The degree of protection is largely dependent upon which functions of the first system are duplicated in the second system. Generally, the protection is highest when all functions are duplicated. For the sake of example, a common temperature sensor could feed two control systems. This would give essentially complete protection except in the event the sensor failed. Separate sensors would avoid this problem.
The duplication of the various system functions is often termed redundancy and the degree of protection offered is therefore largely dependent upon the degree of redundancy.
Complete redundancy essentially doubles the cost of the overall control system. For many applications, this degree of protection is neither economically feasible nor necessary, considering the very low failure rate of components operating at lower voltages (e.g., 12 volts DC).
Therefore, in a control system using electrical heating, failure is largely confined to the switching components operating at supply line voltages. Failure is particularly common in such components as triacs, silicon controlled rectifiers, and some components, such as optocouplers, used for triggering these devices.
Failure is generally of two types; permanent failure as a result of the thyristor shorting from a voltage or current surge, such as might be caused from voltage induced in a power line by a nearby lightning stroke; temporary malfunction from repeated voltage spikes on the power line causing continual firing of thyristors through the dv/dt effect. This temporary malfunction disappears when the repeated spikes cease.
A disadvantage of an overall control system where a second control system is set to control at slightly higher temperature than the first is, that in the event of failure of the first system, the temperature will rise to the point where the second system will control it. In some instances, even a small temperature rise may be detrimental. This may be true, for example, in the case of enzyme reactions.
Another possible disadvantage of some redundant systems is that, where the heating current switching device in the redundant system is of the same nature (e.g., thyristor) as that of the primary system, the same voltage surge, voltage spikes, or any other phenomenon detrimental to one could cause simultaneous failure of the other system.
It is, therefore, an object of this invention to provide relatively low-cost redundant means to protect a thermal system from temperature runaway in the event of failure or malfunction of an output device.
An additional object of this invention is to provide such protection without appreciable rise in the controlled temperature.
Another object of this invention is to provide power switch or power cutoff means of a different nature than those means in the primary system. This will normally be a relay or a circuit breaker with an auxiliary trip coil or the like. In this way, the long switching life of the thyristor is combined with the high transient overload and interference - rejection capabilities of a relay or circuit breaker.